WASHINGTON — As top Democrats offered a public show of support for President Joe Biden throughout the weekend, his advisers worked privately Sunday to push back on suggestions that he step aside, with his campaign manager describing the mechanics of replacing him on the ticket as messy and impractical. 

During a tense call with a group of about 40 of Biden’s top financial backers, Julie Chavez Rodriguez laid out what could and could not be done with the campaign’s infrastructure if…